Making crust:
Butter a 9x13 dish. Line with parchment paper with enough overhang to lift the bars out after they are done baking.
Grate butter on a cheese grater with large holes (this takes a while). In separate bowl, whisk together flour, powdered sugar, and salt. Add butter and stir (I used my hands) til combined and looks crumbly. Transfer mix to baking dish and press evenly onto bottom with your hands. Chill crust in freezer for 15 min. Preheat oven to 350. Bake crust til slightly golden, about 18 min.

Filling:
While crust is baking in a large measuring cup whisk together eggs, sugar, flour and salt til smooth. Stir in lemon juice and milk (I didn't have whole milk so I used 1%). Remove crust from oven and pour filling mix over hot crust. Reduce oven temp to 325 and bake til filling is set and edges are slightly golden brown, about 18 min. Let cool completely on a wire rack. Grip paper; lift out dessert and set on cutting board. Dust with powdered sugar and cut into square. Keep in fridge up to 2 days.